The ammonothermal method is a method for producing a desired material by utilizing the dissolution-precipitation reaction of a raw material using a nitrogen-containing solvent such as ammonia in a supercritical state and/or a subcritical state. When the ammonothermal method is applied to crystal growth, a supersaturated state due to a temperature difference is generated by utilizing the temperature dependence of the solubility of a raw material in a nitrogen-containing solvent such as ammonia, whereby a crystal is deposited.
Specifically, a raw material such as a nitride polycrystal or a seed crystal is placed in a reaction vessel, for example, a pressure resistant vessel such as an autoclave or a capsule, and then, the reaction vessel is hermetically sealed, followed by heating with a heater or the like which is installed inside or outside the reaction vessel, thereby forming a high temperature region and a low temperature region in the reaction vessel, and the raw material is dissolved in one region and a crystal is grown in the other region, whereby a desired crystal is produced.
The nitride polycrystal of GaN or the like to be used as the raw material has an extremely low solubility in a nitrogen-containing solvent such as ammonia in a supercritical state, and therefore, in order to accelerate the crystal growth by improving the solubility, a mineralizer is generally added.
The mineralizer is classified into an acidic mineralizer represented by an ammonium halide NH4X (X=Cl, Br, or I) and a basic mineralizer represented by an alkali amide XNH2 (X=Li, Na, or K). It is known that the use of an acidic mineralizer as the mineralizer has an advantage that the contamination with an alkali metal impurity which impedes the production of a device is prevented, and the production can be performed using a reaction vessel in which a noble metal is used in an inner wall.
PTL 1 and PTL 2 describe a method for producing a nitride single crystal using an acidic mineralizer. In these literatures, as the acidic mineralizer, ammonium chloride, ammonium iodide, and ammonium bromide are exemplified, and it is described that a GaN crystal is grown under the conditions of 650 to 850° C. and 40 to 250 MPa.
However, there is no description of a mineralizer containing a fluorine atom, and moreover, it is described that even when ammonium chloride was used as the mineralizer under a low pressure condition of 27 MPa or 96 MPa, a GaN crystal could not be grown.
Further, PTL 3 describes that a GaN crystal is grown by using ammonium fluoride as a mineralizer containing a fluorine atom. In this method, a crystal is grown under the conditions of 550 to 3000° C. and 500 to 8000 MPa (5 kbar to 80 kbar), and it is essential to perform this method under high pressure conditions.
On the other hand, the growth of a gallium nitride (GaN) crystal by the ammonothermal method is a reaction in a supercritical state at a high temperature and a high pressure (for example, 500° C. or higher and 150 MPa or more), and therefore, the design of an apparatus and the selection of a material to be made so as to withstand such an environment are naturally restricted. Further, as described above, in order to accelerate the crystal growth by improving the solubility of GaN, a mineralizer is generally added.
The environment of a nitrogen-containing solvent in a supercritical state and/or a subcritical state containing such a mineralizer is a very harsh corrosive environment. Therefore, it is necessary to design an apparatus and select a material such that the apparatus and the material have sufficient corrosion resistance even in such an environment.
For example, in the growth of a GaN crystal by the ammonothermal method using an acidic mineralizer, it is possible to increase the purity of the crystal by using Pt in a reaction vessel. However, since Pt is expensive, it is necessary to design the apparatus using another material if the production is tried to be industrially performed at a reduced cost.
Therefore, it has been proposed that as a lining material for a capsule to be used in the apparatus, Cu, Ag, Mo, Fe, or Ta is used, or as a material for an autoclave, Ti, Fe, Co, Cr, or Ni is used, and so on (see PTL 3 to PTL 8).